Officials say that 18 dead birds tested positive for West Nile virus and were found in the following counties: Barron, Brown, Kenoshe, La Crosse, Milwaukee, Oneida, Portage, Rock, Sheboygan and Winnebago.

In July, the virus was detected in three dead birds in Dane, Pierce and Washington counties; adding to a total of 21 dead birds that tested positive for West Nile.

Officials say that infected birds represent an early warning that the virus is present in an area and residents should be careful and protect themselves against mosquito bites.

In 2011, three Wisconsin residents developed symptoms of the West Nile virus.